This invention relates to a process for preparing polyglycerol. In more detail there is provided a process for preparing polyglycerol by treating glycerol under well-defined conditions of temperature and pressure in the presence of a catalytic amount of a system of adjuvants capable of promoting the polymerization reaction. The adjuvant system comprises sulfuric acid and a lower aliphatic acid ester of glycerol. When from about 25 to about 75 percent of the glycerol is polymerized, the glycerol polymerization is terminated by inactivation of the adjuvant system. This can be done by neutralizing the adjuvants with a substantially stoichiometric amount of a neutralizing agent. Unreacted glycerol and cyclic diglycerol are then removed by distillation. The polyglycerol mixture so prepared contains a high percentage of linear di- and triglycerols and a low percentage of polyglycerols comprising seven or more glycerol units. The polyglycerol mixture can be esterified with a fatty acid in known manner to provide polyglycerol fatty acid esters having a polyglycerol distribution which is substantially similar to that of the polyglycerol prior to esterification. These esters are particularly suitable for use as emulsifiers in food applications.
Polyglycerols and the corresponding esters of these polyglycerols and higher fatty acids are known food additives, especially emulsifiers, and also have been used for various food applications. For example, polyglycerol fatty acid esters have been used as anti-spattering agents in cooking and salad oils as disclosed in U.S. Pat. Nos. 3,415,658 and 3,415,659. U.S. Pat. No. 3,528,823 relates to a fluid shortening containing a polyglycerol fatty acid ester emulsifier.
It is known that polyglycerol fatty acid esters can be prepared by polymerizing glycerol in the presence of an alkaline or acid condensation catalyst. The esterification of the polyglycerols so prepared with fatty acids in the presence of an esterification catalyst is also known. The catalytic polymerization of glycerol to polyglycerol is, for instance, described in U.S. Pat. No. 2,487,208 to Alsop. The polymerization is carried out under atmospheric pressure at an elevated temperature, for example, from 200.degree.C to 280.degree.C. Alkali metal and alkaline earth metal compound condensation catalysts can be used. Canadian Pat. No. 834,214 to Kurt et al. discloses the preparation of monoesters of polyglycerol and fatty acids whereby the alkaline condensation catalyst employed is not merely deactivated, but is essentially completely removed from the condensation reaction mixture prior to esterification. For that purpose, the condensation reaction mixture is contacted with both anionic and cationic exchange resins.
U.S. Pat. No. 2,182,397 to Eckey relates to a process for forming ether derivatives of polyhydric alcohols including derivatives that contain esterified fatty acid groups. The polyhydric alcohol, e.g. glycerol, is partially esterified with a carboxylic acid containing two or more carbon atoms. The etherification (polymerization) is carried out in presence of a water carrier and an esterifying catalyst such as sulfuric acid and certain aromatic sulfonic acids. It is also mentioned that the acid catalyst can be neutralized.
While the art is crowded and diverse, it fails to recognize that polyglycerol mixtures having a particular distribution of highly functional emulsifier precursors can be prepared by polymerization of glycerol in the presence of adjuvants capable of controlling the polymerization reaction. It has been disclosed that the neutralization of an alkaline condensation catalyst is deficient as presumably traces of the alkaline material escape neutralization and accordingly are present during the subsequent esterification reaction. The continued polymerization of glycerol and lower polyglycerols to higher polyglycerols is thereby promoted. It is observed that in polymerizing glycerol in the presence of an alkaline catalyst, acid by-products are formed. During the condensation process these by-products react with the alkaline catalyst to form organic salts which cannot be easily deactivated or removed and which act as condensation and interesterification catalysts.
It is an object of this invention to provide a process for preparing polyglycerol compositions which can advantageously be used for the preparation of highly functional polyglycerol fatty acid esters.
It is an additional object of this invention to provide a process for preparing polyglycerol compositions containing a high percentage of lower polyglycerols and a low percentage of polyglycerols comprising seven or more glycerol units.
Other objects of the invention will become apparent from the description of the invention provided hereinafter.